The present invention provides for methods and compositions for inhibiting the polymerization of vinyl monomers, such as olefins and diolefins.
Common industrial methods for producing vinyl monomers include a variety of purification processes such as distillation to remove impurities. Purification operations are often carried out at elevated temperatures and this can increase the rate of undesired polymerization. Polymerization, such as thermal polymerization, during the monomer purification process, results not only in loss of desired monomer end-product, but also in production efficiency caused by polymer formation and deposition on process equipment. Undesirable polymerization causes monomer loss, and may cause operational problems such as increase in fluid viscosity, temperature, restricted flow in pipelines, and block filters. In heat requiring operations, such deposition adversely affects heat transfer efficiency.
Typically the monomers are stabilized with the addition of substances which will act as inhibitors or retarders of polymerization.
Certain vinyl monomers such as the diolefins butadiene and isoprene will polymerize when left in storage tanks and during transportation at temperatures as low as room temperature. This polymerization is initiated by reaction of the diolefin monomer with oxygen present in the monomer containing system. This reaction will form peroxides and free radical species which will perpetuate the reaction with the diolefin monomer.
Various approaches have been attempted with regard to this problem of polymerization. U.S. Pat. No. 3,148,225 teaches that N,N-dialkylhydroxylamines will inhibit the polymerization of popcorn polymer formation in olefin monomer recovery systems. In comparative studies, p-aminophenol was less effective than the hydroxylamines at inhibiting popcorn polymer formation. U.S. Pat. No. 6,200,461 teachs the use of combinations of aminophenols with dialkylhydroxylamines or phenylenediamines. U.S. Pat. No. 3,342,723 tests p- and o-aminophenols for inhibiting fouling of hydrocarbon liquids. These compounds proved effective at inhibiting the formation and adhesion of coke-like deposits during refinery operations.
U.S. Pat. No. 5,510,547 teaches that a combination of a phenylenediamine compound and a hydroxylamine compound is effective at inhibiting the polymerization of vinyl aromatic monomers during processing conditions. U.S. Pat. No.4,720,566 teaches that a combination of a hydroxylamine and a phenylenediamine compound is effective at inhibiting the polymerization of acrylonitrile during its production.
The use of 2,2,6,6-teteramethylpiperidine-N-oxyl (nitroxyl radical) based stable free radicals for controlling free radical polymerization of reactive monomers is well established in literature. U.S. Pat. No. 3,747,988 teaches its use for controlling acrylonitrile polymerization, U.S. Pat. No. 3,733,326 teaches its use for stabilizing vinyl monomers, U.S. Pat. No. 3,488,338 teaches its use for short-stopping the polymerization of chloroprene, U.S. Pat. No. 4,670,131 claims the use of nitroxyl radicals in the range of 20 ppb to 700 ppm for controlling fouling of vinyl monomers.
TEMPO-based nitroxyl radicals are relatively expensive. Synergistic combinations of nitroxyl radicals with other compounds have benefits (economic and technical) and there are a number of patents that teach these types of combinations. Examples are; U.S. Pat. No. 5,711,767 for the use of nitroxyl radical molecules with phenylenediamines to prevent gum formation in gasoline, U.S. Pat. No. 5,888,356 for the use of nitroxyl radicals with nitrosophenols for stabilizing vinyl monomers, U.S. Pat. No. 5,728,872 for the use of nitroxyl radicals with dihetero-substituted benzene for stabilizing acrylic acid, and U.S. Pat. No. 5,254,760 for the use of nitroxyl radicals with aromatic nitro compounds for stabilizing vinyl aromatic monomers. U.S. Ser. No. 09/862,406 teaches the use of nitroxyl compounds in combination with aminophenols to inhibit polymerization of vinyl monomers.